Mass flow sensing semiconductor devices are known for measuring the flow of gas or liquid across a sensing surface using the thermo-transfer (calorimetric) principle. Such devices can be used for direct measurement in the flow range of 0 to 2 slpm (standard liters per minute). One type of semiconductor flow sensor uses a thermal process that utilizes a heating element and two differentially arranged thermocouples to measure the temperature gradient as an indicator of the flow rate.
Flow sensor semiconductor devices can be packaged using MEMS (micro-electromechanical systems) technology. The flow sensing semiconductor device (semiconductor sensor die) can be assembled onto a substrate and packaged along with several other sensor packages in close proximity. The substrate provides electrical interconnections to the semiconductor sensor die. The assembled flow sensor packages are separated after manufacture such that they can be individually implemented into various systems or circuit boards.
In such flow sensor packages, the flow sensor semiconductor device (heating element and thermocouples) is integrated completely inside the package with an active sensing area of the semiconductor die being exposed for measuring the temperature gradient of the flowing gas or liquid. Hence the gas or liquid touches only the durable protective layer of the active sensing area, thereby protecting the semiconductor die from contamination, condensation and abrasion. Also, it is important that the flow sensor package be designed so as to ensure that mass flow along the active sensing area of the die without any turbulence. As discussed in greater detail below, according to the prior art it is known to use a spacer to align the housing of the mass flow package with the level of the die surface to prevent the mass flow from contacting the side wall of the die, thereby minimizing turbulence induced by the die thickness. However, the spacer top surface parallelism to the die surface must be carefully maintained and the bond line thickness of the spacer attachment and die attach is critical. which gives rise to issues during manufacturing and long term performance of the mass flow sensor package.
Moreover, since the die is directly attached to the top surface of the substrate, the wire bonding between substrate and die must encompass the die thickness from the die top surface to the top surface of the substrate, resulting in long wire lengths and high wire loop heights.